Elisabet Rudé

Polydispersity index from linear viscoelastic data: unimodal and bimodal linear polymer melts

Abstract This article describes a method for determining the polydispersity index I p 2 = M z / M w of the molecular weight distribution (MWD) of linear polymeric materials from linear viscoelastic data. The method uses the Mellin transform of the relaxation modulus of a simple molecular rheological model. One of the main features of this technique is that it enables interesting MWD information to be obtained directly from dynamic shear experiments. It is not necessary to achieve the relaxation spectrum, so the ill-posed problem is avoided. Furthermore, a determinate shape of the continuous MWD does not have to be assumed in order to obtain the polydispersity index. The technique has been developed to deal with entangled linear polymers, whatever the form of the MWD is. The rheological information required to obtain the polydispersity index is the storage G′(ω) and loss G″(ω) moduli, extending from the terminal zone to the plateau region. The method provides a good agreement between the proposed theoretical approach and the experimental polydispersity indices of several linear polymers for a wide range of average molecular weights and polydispersity indices. It is also applicable to binary blends.

Viscoelastic properties in the course of hydrolysis and condensation reactions of modified titanium alkozides leading to gelation

Abstract The main goal of this work has been to monitor the sol-gel process through the relaxation time spectrum, H(τ). The system analysed was the polymerization of titanium alkoxo-acetate sols. Four different starting polymeric sols were prepared with different contents of tetraisopropylorthotitanate, acetic acid, isopropanol, water and nitric acid. Continuous dynamic rheological measurements were performed up to, through and after the gel point on a controlled stress rheometer. From the experimental dynamic shear moduli obtained, G′ (ω) and G″ (ω), the relaxation time spectrum was calculated by two methods: the “Smoothed Solution” and the Maximum Entropy methods. The Maximum Entropy method appeared to be more powerful and flexible than the “Smoothed Solution” method. It has been concluded that the gel state can be defined by a linear relaxation spectrum in a log-log plot, independently of the relaxation times.

Rheological Gel Point Determinations in Silica and Titanium Based Sol-Gel Systems

This work deals with the characterization of the gel point (GP) from rheological data in the sol-gel transition. The main purposes of this work have been: (a) to develop a method for the GP determination of gelling systems with universal validity, (b) to analyze the composition dependence of the GP for two different kind of polymeric sols: silica and titanium based sol-gel systems.

Effects of inorganic nitrogen (NH4Cl) and biodegradable organic carbon (CH3COONa) additions on a pilot-scale seawater biofilter

Biofilters degrade a small fraction of the natural organic matter (NOM) contained in seawater which is the leading cause of biofouling in downstream processes. This work studies the effects of chemical additions on NOM biodegradation by biofilters. In this work, biofiltration of seawater with an empty bed contact time (EBCT) of 6 min and a hydraulic loading rate of 10 mh(-1) reduces the biological oxygen demand (BOD7) by 8%, the dissolved organic carbon (DOC) by 6% and the UV absorbance at 254 nm (A₂₅₄) by 7%. Different amounts of ammonium chloride are added to the seawater (up to twice the total dissolved nitrogen in untreated seawater) to study its possible effect on the removal of NOM by a pilot-scale biofilter. Seawater is amended with different amounts of easily biodegradable dissolved organic carbon (BDOC) supplied as sodium acetate (up to twice the DOC) for the same purpose. The results of this work reveal that the ammonium chloride additions do not significantly affect NOM removal and the sodium acetate is completely consumed by the biofiltration process. For both types of chemical additions, the BOD₇, DOC and A₂₅₄ in the outlet stream of the biofilter are similar to the values for the untreated control. These results indicate that this biofilter easily removes the BDOC from the seawater when the EBCT is not above 6 min. Furthermore, nitrogen does not limit the NOM biodegradation in seawater under these experimental conditions.

Relaxation Spectrum Evolution in Polymerization Reactions

The evolution of the viscoelastic properties from liquid-like to solid-like behavior in a sol-to-gel transition has been measured starting immediately after mixing and proceeding up to, through and after the gel point. In this study the attention has been particularly focused on the time evolution of the relaxation time spectrum, H(λ,), since it is the function that characterizes the material in a most general way and its analysis yields information on the molecular weight distribution (MWD) and related properties (Llorens, 1993; Mead, 1996). Therefore, the main purpose of this work has been to characterize polymerization processes from the relaxation time spectra point of view.